Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
ASSETS
Cash equivalents	$ 22,156,000	$ 13,000,000
Deposits with clearing organizations	22,975,000	31,456,000
Securities owned:
Securities owned	952,468,000	735,393,000
Investments	7,243,000
Loans held for sale (3)	1,743,000	60,234,000
Securities purchased under agreements to resell		206,499,000
Derivative contracts:
Derivative contracts	15,660,000	15,609,000
Total	1,015,160,000	1,062,348,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	259,032,000	126,493,000
Derivative contracts:
Derivative contracts	13,605,000	14,162,000
Total	272,637,000	140,655,000
Future [Member]
Derivative contracts:
Derivative contracts	950,000	249,000
Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	6,000	2,000
To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	1,296,000	6,448,000
Derivative contracts:
Derivative contracts	12,507,000	11,619,000
Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts	13,453,000	9,161,000
Derivative contracts:
Derivative contracts		923,000
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	911,000
Derivative contracts:
Derivative contracts	142,000	1,369,000
Investments [Member]
Securities owned:
Investments	158,000	157,000
Equity Securities [Member]
Securities owned:
Securities owned	38,612,000	34,475,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	43,127,000	41,378,000
Money Markets [Member]
Securities owned:
Securities owned	495,000	35,000
Auction Rate Securities [Member]
Securities owned:
Securities owned	89,101,000	86,802,000
U.S. Treasury Securities [Member]
Securities owned:
Securities owned	593,897,000	436,533,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	179,554,000	75,653,000
US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	22,902,000	71,416,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	8,000	15,000
Sovereign Obligations [Member]
Securities owned:
Securities owned	32,000	1,665,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value		1,817,000
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	40,311,000	16,138,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	23,811,000	1,652,000
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	3,875,000	3,504,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	27,000	27,000
Municipal Bonds [Member]
Securities owned:
Securities owned	106,661,000	30,132,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	0	0
Convertible Bonds [Member]
Securities owned:
Securities owned	56,582,000	54,693,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	12,505,000	5,951,000
Level 1 [Member]
ASSETS
Cash equivalents	22,156,000	13,000,000
Deposits with clearing organizations	22,975,000	31,456,000
Securities owned:
Securities owned	637,519,000	496,283,000
Derivative contracts:
Total	682,650,000	540,739,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	222,681,000	117,031,000
Derivative contracts:
Derivative contracts	956,000	251,000
Total	223,637,000	117,282,000
Level 1 [Member] | Future [Member]
Derivative contracts:
Derivative contracts	950,000	249,000
Level 1 [Member] | Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	6,000	2,000
Level 1 [Member] | Equity Securities [Member]
Securities owned:
Securities owned	38,612,000	34,475,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	43,127,000	41,378,000
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities owned	495,000	35,000
Level 1 [Member] | U.S. Treasury Securities [Member]
Securities owned:
Securities owned	593,897,000	436,533,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	179,554,000	75,653,000
Level 1 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	4,515,000	25,240,000
Level 2 [Member]
Securities owned:
Securities owned	225,823,000	152,227,000
Loans held for sale (3)	1,743,000	60,234,000
Securities purchased under agreements to resell		206,499,000
Derivative contracts:
Derivative contracts	1,296,000	6,448,000
Total	228,862,000	425,408,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	36,351,000	9,462,000
Derivative contracts:
Derivative contracts	12,507,000	11,619,000
Total	48,858,000	21,081,000
Level 2 [Member] | To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	1,296,000	6,448,000
Derivative contracts:
Derivative contracts	12,507,000	11,619,000
Level 2 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	18,387,000	46,176,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	8,000	15,000
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities owned	32,000	1,665,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value		1,817,000
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	40,311,000	16,138,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	23,811,000	1,652,000
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	3,875,000	3,504,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	27,000	27,000
Level 2 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	106,636,000	30,051,000
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities owned	56,582,000	54,693,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	12,505,000	5,951,000
Level 3 [Member]
Securities owned:
Securities owned	89,126,000	86,883,000
Derivative contracts:
Derivative contracts	14,364,000	9,161,000
Total	103,648,000	96,201,000
Derivative contracts:
Derivative contracts	142,000	2,292,000
Total	142,000	2,292,000
Level 3 [Member] | Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts	13,453,000	9,161,000
Derivative contracts:
Derivative contracts		923,000
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	911,000
Level 3 [Member] | Investments [Member]
Securities owned:
Investments	158,000	157,000
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities owned	89,101,000	86,802,000
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	142,000	1,369,000
Level 3 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	25,000	81,000
OMHHF
Derivative contracts:
Securities owned	$ 562,000	$ 562,000